During the operation of a hydrocarbon well (i.e. a gas or oil well) various down-hole problems can arise including the deposition of scale which inhibits the hydrocarbon flow. Scale is a water-related problem which arises as a result of the commingling of incompatible aqueous fluids in the formation (i.e. the rock). For example, where sea water is injected into a subterranean formation to drive oil through the formation into a producer well hole, differences in the nature of the ions present in the injection water and that already present in the formation may cause the precipitation of metal salts. In the North Sea, typical scale problems are related to the production of inorganic salts such as BaSO4, SrSO4, CaSO4 and CaCO3. These salts precipitate as scale which, if left untreated, causes scaling of subsurface and surface production equipment and/or tubing and, eventually, blockage of the well hole. Commingling of incompatible aqueous fluids usually occurs within the near well bore area of a subterranean formation. The severity of the problem is highly dependent on the field operating conditions, which can vary from mild scaling tendencies to the extreme.
To prevent scale from forming in the system, a chemical inhibitor is typically injected continuously and/or by periodic or intermittent treatments. The scale inhibitor prevents the formation of scale thereby increasing oil or gas flow. One type of intermittent treatment is so-called “squeeze” treatments. Indeed in the case of reservoir treatments intended to protect the critical near well bore area, squeeze treatments are normally preferred.
In a squeeze treatment, a scale inhibitor at concentrations between 5-20% by weight is normally injected into the formation through a producer well hole after a pre-flush. After over-flush and shut-in, well production is then resumed. Ideally the production water then slowly leaches or washes out the retained scale inhibitor from the formation. More specifically the leaching process should place a low, but still effective, concentration (e.g. around 1-100 ppm) of the scale inhibitor in the produced water to prevent scale deposition. Depending on the inhibitor retention and release properties in the formation, however, the effect of this treatment may last from one month to about 24 months. For economic reasons, a prolonged period of protection from scale formation is clearly desirable.
An ideal scale inhibitor return curve for scale inhibitor concentration is one where, after the overflush is complete, the inhibitor desorbs into the produced water at a rate that provides a constant concentration that is the minimum required to prevent scale formation, i.e. the minimum inhibitory concentration (MIC). Even more ideally, this process continues until all of the scale inhibitor squeezed into the formation is released in this way.
Typically, however, squeeze treatments do not provide ideal scale inhibitor return curves. Usually the concentration of scale inhibitor in the produced water is initially high, and much greater than that required to prevent scale formation, as a result of inhibitor failing to adsorb or attach to the formation. Thereafter the concentration of scale inhibitor tends to decrease until it eventually falls below the minimum required to prevent scale deposition. The process can therefore be inefficient as a large proportion of the inhibitor introduced in the squeeze treatment is returned almost immediately and does not serve to prevent scale formation. Moreover regular repetition of scale inhibitor treatment is highly undesirable as oil production invariably needs to be stopped to allow the treatment to be carried out.
Various techniques have been developed to try to increase the length of time for which a squeeze treatment provides an effective concentration of scale inhibitor in the formation. For example, WO2004/011772 and WO2008/020220 disclose the use of “bridging agents” which are positively charged polymers. These polymers are used to precondition a rock material and thereby enhance retention of a scale inhibitor thereto. The polymers disclosed as possible bridging agents are polyaminoacids such as polyaspartate and polymers formed from diallylmethylammonium chloride. It is thought that the use of charged polymers such as those described in these publications enhance retention of scale inhibitors in subterranean formations by a mechanism wherein the adsorption of the positively charged compounds to the formation reduces its negative charge. As a result scale inhibitors, which are often negatively charged, are more readily retained on the formation.
The use of microemulsion-based scale inhibitors has also been suggested for increasing the retention of scale inhibitor to subterranean formations. It is thought that the use of such emulsions may increase treatment lifetime by the oil phase of the emulsion being able to displace organic material from formation surfaces thereby making more of it available for the scale inhibitor to adsorb onto. This approach has the advantage of being relatively simple since it involves no additional preconditioning step. Unfortunately, however, microemulsions are relatively expensive to make compared to conventional squeeze treatment agents.
Other strategies for enhancing inhibitor retention in hydrocarbon wells have focussed on modification of the inhibitor itself, rather than on provision of additional agents. It has been reported, for example, that inhibitor retention in oil wells may be enhanced by cross-linking scale inhibitors, e.g. by ester cross-linking of polycarboxylic acid scale inhibitors. In this method the molecular weight of the cross-linked scale inhibitor increases the molecular weight of the scale inhibitor so that stronger adsorption to the formation surface may be achieved.
WO03/106810 discloses a process based on this principle. In WO03/106810 it is disclosed that size-controlled microparticles of cross-linked scale inhibitor having a mean particle diameter of less than 10 microns may be formed under conditions of high shear or by comminution of a dried macrogel comprising cross-linked scale inhibitor. When such particles are injected into a formation, the scale inhibitor is released by hydrolysis of the ester cross-links. Since the rate of hydrolysis is dependent upon the conditions the particles encounter (e.g. pH, temperature, pressure), in the well the rate at which scale inhibitor is released may vary. Under appropriate conditions, such a strategy may increase the length of time for which an effective concentration of scale inhibitor may be provided in a subterranean formation by providing scale inhibitor having a higher molecular weight as well as by a slower release of inhibitor.
Nevertheless some types of formation are still difficult to effectively treat economically, especially by squeeze treatment methods. This is, for instance, often the case with clean sandstone formations having a low clay content, especially if they also have a low permeability or zones having a wide range of permeabilities. Lower permeability formations (or formations having zones of lower permeability) are more complicated to treat than higher permeability formations as great care needs to be taken not to further reduce permeability as a consequence of the treatment. The treatment of such formations with very high molecular weight treatment agents (e.g. cross-linked polymer) may therefore be impossible due to the risk that pore throat blockage occurs.
Hence there is still a need for alternative methods for inhibiting scale formation within a hydrocarbon producing system, and in particular, for methods that increase the retention of scale inhibitors in oil wells. In addition the methods should not damage the formation (e.g. reduce its permeability) so that it is applicable to lower permeability formations and any agents employed should preferably exhibit good biodegradation properties with low toxicity and low bioaccumulation. In light of increased environmental concerns, it is essential that this requirement be met in order for the method to be commercially useful.
It has now been found that certain minerals, and in particular clay minerals such as kaolinite, are particularly suitable for use in conjunction with an organosilane in methods of increasing the retention of scale inhibitors in subterranean formations. Surprisingly such methods have been found to be especially effective when a metal carbonate such as CaCO3 is additionally used. These methods have been found to be increase the length of time for which a squeeze treatment is effective.